The present invention relates to an image recording device of the ink jet type which ejects ink drops through the nozzles thereof to record an image on a recording medium.
An image recording device of the type in which the ink tank for supplying ink to the recording head or printhead thereof is detachably attached to the recording device per se has been developed and is currently marketed. In this type of the image recording device, only the replacement of the ink tank with a new one suffices for the ink supply to the printhead. This ink tank may be manufactured at low cost. The result is the reduction of the running cost of the recording device.
In this ink supplying system using the detachable type ink tank in the image recording device, a filter for preventing the ink leakage in used in the part of the ink tank in the jointing portion between the ink tank and the printhead. Another filter is also provided in the part of the printhead in the jointing portion. The filter prevents dust particles and the like from entering the printhead when the ink tank is removed from the printhead.
In an image recording device disclosed in the Unexamined Japanese Patent Application Publication No. Hei. 6-71900, a first filter is provided at the ink inlet of the printhead, and a second filter is provided at the ink supplying port of the ink tank. The mesh size of the first filter is selected to be larger than that of the second filter. With the construction, when the ink tank is attached to the printhead, air bubbles left in the ink passage are led to the printhead, whereby the flow of air bubbles into the ink tank is checked. The air bubbles are sucked out of the printhead through the nozzle by the ink suction in a maintenance, for example. Sometimes, the air bubbles are still left in the printhead. In this case, the air bubbles are present in the ink passage, possibly causing improper discharging of ink. The improper ink discharging gives rise to a picture defect of the printed picture.
Another image recording device is disclosed in the Unexamined Japanese Patent Application Publication Nos. Hei. 8-224884 and Hei. 8-207298. In each publication, the ink supplying system guides to the ink tank air bubbles that Are left in the ink passage after the ink tank is attached to the printhead while checking the flowing of air bubbles to the printhead. Thus, in those publications, the destination of the residual air bubbles is the ink tank while it is the printhead in the publication already referred to, the Unexamined Japanese Patent Application Publication No. Hei. 6-71900. So far as we read, there is no description on the conditions of the filters. In the ink supplying system of each of the recording devices of those publications, i.e., the Unexamined Japanese Patent Application Publication Nos. Hei. 8-224824 and Hei. 8-207298, if the mesh size of the filter placed at the ink inlet of the printhead is smaller than that of the filter at the ink supplying port of the ink tank, the air bubbles left In the jointing portion must move to the ink tank. Therefore, the air bubbles entering the printhead is considerably reduced in amount, so that a frequency of the occurrence of the improper discharging of ink, which is due to the air bubbles, is reduced.
Thus, those filters have functions to remove foreign material from the ink in the ink tank and to check the entering of foreign materials into the printhead. FIG. 7 is a table showing, by way of example, relationships between filtering particle sizes and particle passing efficiencies of filters. In the table of FIG. 7, mat figured cloth filters of different mesh sizes are shown. The mesh sizes of those filters are 12 .mu.m, 13 .mu.m and 30 .mu.m. Foreign materials of different particle sizes are used. The particle sizes of the foreign materials are 10 .mu.m, 20 .mu.m, 30 .mu.m and 40 .mu.m. The table describes those foreign materials that passed through those filters in terms of %. The mat figured cloth filter of 12 .mu.m in mesh size substantially rejects the passing of foreign materials of 40 .mu.m particle size. The remaining foreign materials of 20 .mu.m, 30 .mu.m, 40 .mu.m, which passed through the filter are: 55%, 10% and 3%. The foreign materials of 10 .mu.m, 20 .mu.m, 30 .mu.m, 40 .mu.m, which passed through the mat figured cloth filter of 38 .mu.m in mesh size, are 96%, 80%, 63% and 50%. As seen from the table, if highly precise filters of small mesh size are used, it is possible to increase the efficiency of arresting foreign materials contained in the ink within the ink tank and to reduce the number of foreign materials in the ink supplied to the printhead. The result in to reduce a frequency of the occurrence of the improper ink discharging, which is caused by foreign materials, e.g., dust particles, and hence to stably record a quality picture on the recording medium, e.g., a printing paper.
In the ink supplying system of the type in which foreign materials are arrested by use of the filters, the mesh of the filter is frequently clogged with foreign materials when the recording device or printer is used for a long time. Particularly where the filter of a small mesh size is used in the part of the printhead as in the above case, fine foreign materials pass through the filter in the part of the ink tank, and are arrested by the filter in the part of the printhead. Therefore, the filter of the print head tend to be clogged with the foreign materials. The filter clogging leads to an increase of a fluid resistance of the filter. If the printer whose fluid resistance is high is operated for a high density printing, an insufficient amount of ink is supplied to the printhead, and air is sucked through the nozzles of the printhead. The resultant picture printed on the printing paper suffers from a picture defect, e.g., bleaching.
FIG. 8 is a graph showing a variation of fluid resistance of a filter against the amount of used ink. For a measurement to gather data depicting the graph, a mat figured cloth filter of 12 .mu.m in mesh size was used in the part of the printhead, and a mat figured cloth filter of 38 .mu.m in mesh site was used in the part of the ink tank. Here, a fluid resistance is defined as R(Pa.multidot.sec/m.sup.3) when P(Pa)=RQ(m.sup.3 /sec). A viscosity of ink used for the measurement was 2.0.times.10.sup.-3 Pa.multidot.sec.
A seen from FIG. 8, a fluid resistance of the filter exceeds a limit resistance value within which a normal printing is possible. Thus, even if the printhead of long lifetime, is used, the printer is unusable because of the filter clogging.